Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and embedded servo sectors. The embedded servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo controller to control the velocity of the actuator arm as it seeks from track to track.
FIG. 1 shows a prior art disk format 1 as comprising a number of servo tracks 3 defined by servo sectors 5O-5N recorded around the circumference of each servo track. Each servo sector 5i comprises a preamble 7 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 9 for storing a special pattern used to symbol synchronize to a servo data field 11. The servo data field 11 stores coarse head positioning information, such as a servo track address, used to position the head over a target data track during a seek operation. Each servo sector 5i further comprises groups of servo bursts 13, which are recorded with precise intervals and offsets relative to the track centerlines. The servo bursts 13 provide fine head position information used for centerline tracking while accessing a data track during write/read operations.
When the disk drive is idle (not servicing host commands), the head is typically parked on a ramp located near an outer edge of the disk. The control firmware typically maintains the state of the head (parked or un-parked) so that the disk drive knows whether the head should be unloaded from the ramp before executing a new host command. However if the control firmware malfunctions when determining the state of the head, the disk drive may fail when attempting to execute a new host command.
It is also typically desirable to detect when the head first contacts the ramp during an unload operation to determine the maximum stroke of the actuator arm. For example, the location of the ramp may be determined prior to self-servo writing a disk so that the servo tracks may be written from the inner diameter of the disk to the edge of the ramp (or very near thereto), thereby maximizing the usable disk space.